FAQ for
8 Mile (2002) More at IMDbPro »

Check out the wikipedia entry on the subject: http://en.wikipedia.org/wiki/Uncle_Tom.

Basically, it's a black man who sucks up to a white man because they are white.

In the movie, BRabbing sees Wink hanging out with the Free World and starts a fight with them. (it's the fight where Cheddar Bob shoots himself) There isn't a clear reason for the motives behind the fight but we know this:

1. Wink said during the Chin Tiki club scene that he won't hang out with the Free World anymore

2. Wink wants a piece of the profit; he knows that Papa Doc and BRabbit have potential, so he hangs around both of them hoping that he can get a piece of the profit.

3. Janeane had told him earlier on that everyone was dissing him behind his back because he choked at the show a few nights before and because of Lyckety-Splyt mocking of him the next night, he could of assumed it was him and the rest of the free world spreading stuff about him.

This probably gave Brabbit reason to start the fight.

Eminem was dissing how gangster his opponents were.